1. Field of the Invention
The subject matter of this invention is related generally to electromagnetic contactors and more specifically to the magnetic armatures associated with such electromagnetic contactors.
2. Description of the Prior Art
Electromagnetic contactors are well known in the art. A typical example may be found in U.S. Pat. No. 3,339,161 issued Aug. 29, 1967 to J. P. Conner et al. entitled "Electromagnetic Contactor" and assigned to the assignee of the present invention. Electromagnetic contactors are switch devices which are especially useful in motor-starting, lighting, switching and similar applications. A motor-starting contactor with an overload relay system is called a motor controller. A contactor usually has a magnetic circuit which includes a fixed magnet and a movable magnet or armature with an air gap therebetween when the contactor is opened. An electromagnetic coil is controllable upon command to interact with a source of voltage which may be interconnected with the main contacts of the contactor for electromagnetically accelerating the armature towards the fixed magnet, thus reducing the air gap. Disposed on the armature is a set of bridging contacts, the complements of which are fixedly disposed within the contactor case for being engaged thereby as the magnetic circuit is energized and the armature is moved. The load and voltage source therefor are usually interconnected with the fixed contacts and become interconnected with each other as the bridging contacts make with the fixed contacts.
Typically, contactors are classified as either DC devices or AC devices. In alternating current contactors magnetic noise is caused by the coil voltage returning to zero at a rate determined by the frequency of the power supply, typically this is 60 hertz. The noise level is reduced by adding shading coils to the magnetic circuit. The shading coil result in a current flow when the voltage is zero. The current flow results in a force that keeps the magnet closed and quiet. It is the movement of the magnet due to the AC power that causes the noise. However, in efficient low-cost, simple, electrical systems it would be advantageous to eliminate the use of a shading coil. In a DC device magnetic noise is not considered a problem because there are no zero voltage crossings. In both AC and DC contactors a non-magnetic gap is often added to the path of the magnetic system to limit the residual magnetism that causes magnetic sticking. When an E-shaped magnet is used in prior art systems an air gap is added to the magnetic path of the center leg by making that leg shorter than the outside legs. This air gap increases the magnetic reluctance of the closed magnetic path thus reducing the residual magnetism thus rendering the kickout spring more effective for separating the magnets during a contactor opening operation. However, in an E-shaped magnetic member in an AC system vibration of the center leg due to the fact that there is some room for movement thereof due to the placement of the air gap therein causes deflection of the spine of the E-shaped member allowing the outer pole pieces to wipe against abutting complementary members of an associated permanent magnet. The latter movement causes the outer leg pole faces to wear which eventually causes the center leg air gap to disappear and the residual magnetism to increase dramatically. In the present system, a periodic holding pulse is provided to the magnetic coil even when the contactor is closed to maintain the contactor in the closed state. This periodic signal causes vibration of the center leg of the E-shaped magnetic member introducing noise due to the vibration of the center leg and the rubbing of the pole pieces of the complementary outer legs against each other. Yet it is still desirable to maintain the air gap. It would be advantageous, therefore, if a magnetic system could be discovered which had the benefits of the central air gap to reduce residual magnetism and yet which prevented vibration of the central leg to eliminate noise and wear.